Ultrasound medical diagnostic systems generate images of anatomical structures within a patient's body by scanning a target area with ultrasound signals. Typically, ultrasound signals on the order of 2 MHz to 10 MHz are transmitted into a patient via a transducer probe. The transmitted ultrasound energy is in part absorbed, dispersed, refracted, and reflected by the patient's body. Reflected ultrasound energy is received at the transducer probe where it is converted into electronic echo signals. The echo signals undergo beam forming to correlate the ultrasound signals. Subsequently, the beam-formed signals, also referred to as beamlines or vectors, are processed to analyze echo, Doppler, and flow information and to obtain an image of the patient's targeted anatomy, such as tissue or blood flow.
Most ultrasound systems operate in an imaging or B-mode that provides a physician or sonographer with an image of the tissue under examination, and a color Doppler mode. A B-mode image, for example, is a brightness image in which component pixels are brightened in proportion to a corresponding echo signal strength. The brightness image represents a cross-section of a patient target area through a transducer's scanning plane.
FIG. 1 shows a typical B-mode ultrasound image 10 that is created by an ultrasound system and displayed to a physician or sonographer. The image 10 includes a two-dimensional picture of internal body matter of a patient. In the example shown, the body matter includes an artery 12 having walls 13 and an amount of blood 14 flowing through it. However, the B-mode ultrasound image 10 does not show a representation of the flow of blood 14 through the artery 12. To create a color Doppler image, color flow information is overlaid onto the B-mode image 10. The color flow image is color or brightness coded so that higher flow velocities appear brighter in the image 16 than lower flow velocities.
Occasionally, a portion of a returned echo signal may be missing due to scattering or absorption from a scatterer that is in the path of the returned echo signal. This causes a "hole" that appears in the color flow image 10 as a dark portion 32. Because low flow velocities are also represented in the color flow image as dark areas, the hole 32 can impair the ability of the sonographer or physician to analyze the image, especially when the hole 32 appears in an area of the color flow image 16 representing a high flow velocity.
FIG. 2A shows a profile 16 of the blood 14 flowing through the artery 12. The flow of the blood 14 in the artery 12 has a profile such that the flow has a low value near the walls 13, increases as the distance from the walls 13 increases, and is at a maximum near the center of the artery 12.
Conventional ultrasound systems attempt to improve color flow sensitivity by performing smoothing. Conventional smoothing averages flow velocity information over a two-dimensional window, or kernel. However, conventional smoothing is performed regardless of flow characteristics. That is, conventional smoothing is either performed on all pixels of a color flow image, or smoothing is not performed at all. As a result, conventional smoothing can result in smearing of ultrasound color flow images. Thus, the sharp flow profile of FIG. 2A is flattened as shown in FIG. 2B as a profile 17, and the resultant color flow image is blurred. Further, conventional smoothing can result in distortion of flow haemodynamics, especially during systole or turbulent flow, and consequently can result in loss of diagnostic information.
The above-mentioned disadvantages of conventional smoothing result because conventional smoothing is performed regardless of flow characteristics. Therefore, there is an unmet need in the art for a method of smoothing an ultrasound color flow image without causing the distortions associated with conventional smoothing techniques.